The problem is not to arrange the sensor if possible at all in the space with the different environmental conditions subsequently generally designated as positive pressure space without limiting the difficult condition to a physical magnitude of the pressure so that damages to the sensor are prevented.
On the other hand side the operating cylinder unit shall require as few as possible additional features possibly no additional features for housing the sensor unit in order not to increase cost of the operating cylinder unit.
On the other hand side arranging portions of the sensor unit on an outside of the cylinder of the operating cylinder unit is disadvantageous since they can be damaged easily in the typically rough environment where the operating cylinder unit are being used for example in construction equipment that is exposed to the elements.
So far the following solutions are typical:
a) infrared ultrasound sensors:
touch free scanning and detecting of a distance of a face of the piston by an infrared ultra sound sensor arranged in an interior of the cylinder at an end.
Sensors of this type are rater sensitive and still expensive and measuring precision is limited.
b) tension element sensors:
a positive pressure cavity in an interior of the cylinder includes a typical measuring cable sensor or a measuring band sensor in which a cable or band that is wound onto a winding drum and preloaded in the wind up direction by a flat spiral spring is attached with the free end at the piston.
The number of revolutions and the angular position of the cable drum are detected and a pull off length of the cable and thus a position of the piston in the cylinder is determined.
The sensor detecting the revolutions and the angular position of the cable drum can also be arranged in the positive pressure cavity which however can lead to damaging the sensor element easily.
The rotation and rotational position of the winding drum however can also be transmitted touch free by a magnet connected to the winding drum through an accordingly configured portion in the cylinder wall to the magnetic field sensitive sensor arranged on an outside of the portion of the cylinder wall.
Thus however the cylinder typically made from a thick steel material has to be provided with a window either with a smaller material thickness or made from another e.g. magnetic field permeable material which increases the fabrication cost of the cylinder by a large amount and also reduces its pressure bearing capacity by a large amount.
c) rod-sensor units of the magneto astrictive type:
The sensor unit made from a head and a sensor rod axially protruding therefrom is installed at an axial end in the cylinder and sealed at an outer circumference relative to the inner circumference of the cylinder so that the sensor rod protrudes towards another end of the cylinder, typically in a center of the cylinder.
The piston rod is bored hollow so that the sensor rod can penetrate into the longitudinal bore hole in the piston rod.
The piston rod supports a typically annular encoder magnet at an end that is oriented towards the sensor head wherein a magnetic field of the encoder magnet influences an electrical signal running in the wave conductor in an interior of the sensor rod through a current axial position of the encoder magnet along the sensor rod and influences a density wave in the wave conductor that runs back to the sensor head. Thus the sensor element arranged in the sensor head and in a subsequent processing unit facilitates detecting an axial position of the encoder magnet and thus of the piston rod relative to the sensor rod and thus relative to the cylinder.
Thus the sensor head typically contacts a shoulder proximal to the cylinder base. The data transfer from the sensor head that is typically performed by a cable is provided by a cable outlet from the sensor head on an ambient pressure side oriented away from the piston and a pass through opening in the cylinder typically arranged in a base of the cylinder.
The required complex hollow boring of the piston rod is a distinct disadvantage.